The present invention relates to a process for producing polyester polyols from isophthalic acid and/or terephthalic acid, oligoalkylene oxides and phthalic acid or phthalic anhydride, the polyester polyols obtainable by this process and the use thereof for producing rigid PUR/PIR foams.
Rigid PUR/PIR foams are produced today predominantly on the basis of polyester polyols, since these have a positive impact on the flame resistance of rigid PUR/PIR foams and their thermal conductivity. As raw materials in the production of polyester polyols, succinic acid, glutaric acid, adipic acid, phthalic acid/anhydride, terephthalic acid and isophthalic acid are primarily used. In addition to the polyester polyols, polyether polyols are occasionally also added to improve the solubility behaviour of pentanes towards the polyester polyols or to reduce the brittleness of isocyanurate-containing rigid PUR/PIR foams.
However, the use of aromatic acids, in particular the use of terephthalic acid, in the production of polyester polyols can lead to these being present in solid form at ambient temperature and thus makes them more difficult to process in industrial processes.
U.S. Pat. No. 4,758,607 discloses, for the production of these polyester polyols, high molecular-weight poly(ethylene terephthalate), PET, as the raw material basis, which is reworked by molecular weight-reducing reaction media, such as e.g. low molecular-weight glycols, also in the presence of low molecular-weight polycarboxylic acids, to form a new polyester polyol. However, it is a disadvantage of such a procedure that the PET first has to be collected in a complex process. Furthermore, it must be ensured that the material is correctly sorted and uncontaminated. Where recycled material is concerned, e.g. PET drinks bottles, for example the caps, which are generally made of poly(ethylene), have to be removed which is time-consuming. Where PET production waste is concerned, this raw material is not universally available but is linked to the presence of a PET production plant. A further disadvantage also consists in the fact that part of the glycol used for PET degradation according to the teaching of U.S. Pat. No. 4,758,607 has to be removed again by distillation, which is disadvantageous in terms of energy in view of the high boiling point of glycols.
U.S. Pat. No. 4,039,487 discloses polyester polyols based on terephthalic acid, tetraethylene glycol and phthalic anhydride. However, U.S. Pat. No. 4,039,487 does not disclose how the disadvantages known to the person skilled in the art of an esterification of these components, i.e. long reaction times as a result of the poor solubility of terephthalic acid, can be overcome. A further disadvantage is that, as a result of the rapid reaction of tetraethylene glycol with phthalic anhydride, the number of hydroxyl groups available for esterification of the terephthalic acid is rapidly reduced right at the start of the reaction, which has an unfavourable effect on the subsequent reaction of the less reactive terephthalic acid, since its rate of esterification is also, among other things, proportional to the concentration of free hydroxyl groups. An alternative was therefore sought to the use of significantly larger amounts of esterification catalysts, since such catalysts can negatively affect subsequent reactions with these polyester polyols, e.g. the production of PUR foams.
It was therefore one of the objects of the present invention to remedy the above-mentioned disadvantages of the prior art.
Many conventional rigid PUR/PIR foams based on polyester polyols, however, still fail to exhibit adequate flame resistance, since they generally only achieve fire protection class B3 according to DIN 4102-1.